1. Field of the Invention
The present invention generally relates to a switching hub used in a local area network (LAN). More specifically, the present invention is directed to a switching hub capable of controlling a communication quality of a LAN.
2. Description of the Related Art
Switching hubs may be recognized as one of very important LAN products in a so-called "ATM-LAN (Asynchronous Transfer Mode-Local Area Network) generation" in view of the below-mentioned technical points. That is, the switching hubs can be directly connected to the existing Ethernet LAN environment, and moreover the switching hubs can have the virtual LAN (VLAN) function. This VLAN function is necessarily required to merge the ATM technique into the existing LAN environment.
However, in such a case that the conventional switching hubs are applied to the multimedia communication, data drops (frame discards) happen to occur in communications in data systems, and also frame delays happen to occur in communications in sound systems.
For instance, as shown in FIG. 22, a terminal "A" used to send out a frame of a data system and another terminal "B" used to send out a frame of a sound system are connected to a conventional switching hub. Considering now such a case that the terminals "A" and "B" transmit frames in different frame intervals and furthermore in an interval substantially equal to such a time period required to switch a single frame of a switch unit.
In this case, a frame "A" sent from the terminal A is saved into an input buffer (port) for the terminal A employed in the switching hub, whereas a frame "B" sent from the terminal B is saved into an input buffer for the terminal B. Then, the switch unit employed in the switching hub judges as to whether or not the frames are saved into the respective input buffers in the round robin manner. When the frame is saved in the input buffer, the frame saved in this input buffer is supplied to an output buffer (namely, port; only one port being shown in this drawing) corresponding to the destination address within the frame. As a consequence, as schematically illustrated in FIG. 22, the frame B in the sound system would be delayed within the switching hub (namely, frame interval 1' would be widened, as compared with frame interval 1). Further, there is such a fact that a total number of frames derived from the respective input buffers per unit time is not controlled in accordance with a total number of frames entered into the respective input buffers. As a result, as indicated in the input buffer for the frame A, the input buffer contains no empty region, so that the frame A would be discarded.
Since the data drops are not allowed in the communication in the data system, when the frame A is discarded, the data is retransmitted. In other words, in such a case that the frame is discarded in the communication in the data system, in order to communicate such data having a certain size, a network resource is used by which data having a larger size than this size by several tens percent is transmitted. On the other hand, even if the data drops more or less occur, it is possible to recognize the contents of this communication in the data receiving terminal during the communication in the sound system. However, when the data is delayed, it is practically difficult to recognize the contents of this communication.
As a consequence, in the conventional switch hub, the following communication quality control is required. That is, the switching operation of the frame A is carried out with a top priority in order not to discard the frame A even though the frame B is more or less discarded. Otherwise, it is desirable that the switching operation of the frame B can be carried out with a top priority in order not to delay the frame B by interrupting the communication of the frame A at an earlier stage. However, the conventional switching hub was not provided with such a communication quality control function.
Also, a router is required so as to form a network constituted by a plurality of LANs by combining the conventional hubs with each other, otherwise with employment of the VLAN function of the switching hub.
For example, as represented in FIG. 23, in order that a network constructed of two sets of LANs is constituted by employing two sets of the conventional switching hubs, a router must be provided between two sets of these switching hubs. In this case, the frames are switched at a high-speed along the terminals stored in the same switching hub, because this frame switching operation is carried out by such a simple process operation called as a "MAC address retrieving" operation. However, when a frame derived from one switching hub is transferred to the other switching hub, various sorts of algorithm processes of the upper-grade layer are performed between terminals, for instance, between the terminal A and the terminal E where the frame should be transmitted via the router. Since the time required for processing 1 frame by a router becomes very longer than the time required for processing 1 frame by a switching hub, normally 50 times long, the communication performance of the communication established between the LANs in the network constituted by employing the conventional switching hubs would be considerably lowered, as compared with that for the communication established within the LAN. In other words, in this network, the high-speed characteristic of the switching hub could not be utilized in the communications established between the LANs.